Impact of Anion Vacancies on the Local and Electronic Structures of Iron-based Oxyfluoride Electrodes

. The properties of crystalline solids can be significantly modified by deliberately introducing point defects. Understanding these effects, however, requires understanding the changes in geometry and electronic structure of the host material. Here we report the effect of forming anion vacancies, via dehydroxylation, in a hexagonal-tungsten-bronze–structured iron oxyfluoride, which has potential use as a lithium-ion battery cathode. Our combined pair-distribution function and density-functional–theory analysis indicates that oxygen vacancy formation is accompanied by a spontaneous rearrangement of fluorine anions and vacancies, producing dual pyramidal (FeF 4 )–O–(FeF 4 ) structural units containing five-fold–coordinated Fe atoms. The addition of lattice oxygen introduces new electronic states above the top of the valence band, with a corresponding reduction in the optical band gap from 4.05 eV to 2.05 eV. This band gap reduction relative to the FeF 3 parent material is correlated with a significant improvement in lithium insertion capability relative to defect-free compound.


KEYWORDS.
Defects, pair distribution function, density functional theory, band gap engineering.
The search for high capacity electrodes for rechargeable lithium-ion batteries has led to many materials being proposed as potential candidates. Iron fluorides contain earth-abundant iron as their redox-active species, and have a high theoretical capacity (237 mA h g -1 for 1 e) and a high voltage vs. Li + /Li of 3.0 V-3.3 V. 1 Metal fluorides, however, are electronic insulators characterized by large bandgaps (> 3 eV), which limits their electrochemical properties due to poor electronic conductivities. To overcome this limitation, several approaches have been proposed, including: (i) the synthesis of nanocomposites containing carbon; 2 (ii) the preparation of oxyfluorinated compounds with reduced band gaps, which can produce complex electrochemical behaviour; 3,4,5 (iii) the stabilization of open-frameworks such as the hexagonal-tungsten-bronze 6 (HTB) and pyrochlore 7 type structures, that can be functionalized with single wall nanotubes (SWNTs) 8 ; (iv) surface engineering to protect the electrode materials 9 ; and (v) the introduction of anionic vacancies 10 in an oxyfluoride HTB-type structure, which can be denoted FeF3-xOx/2ox/2, with o referring to anionic vacancies.
One proven strategy for improving the electrochemical performance of electrode materials is to deliberately introduce defects, such as vacancies. [10][11][12][13] At present, a general understanding of this effect is absent. Building such an understanding requires a detailed characterization of the relevant defects, to allow the coupled changes in stoichiometry, crystal geometry, and electronic structure to be resolved. We have previously reported the stabilization of anionic vacancies in an iron oxyfluoride compound FeF3-xOx/2ox/2, studied using x-ray diffraction analysis and Mössbauer spectroscopy. 10 We previously reported that the substitution of fluoride by oxide anions is accompanied by the formation of anionic vacancies, producing undercoordinated iron atoms. The effect of fluorine substitution by oxygen and accompanying anion vacancy formation on the local structure has yet to be fully assessed. Here, we describe a combined experimental and computational study, performed to resolve the changes in crystal geometry and electronic structure induced by concomitant oxygen substitution and anion vacancy formation.
The vacancy-containing iron oxyfluoride compound FeF3-xOx/2ox/2 was prepared by a two-step thermal decomposition process of FeF3.3H2O: a compound that can be produced from spent liquor from stainless steel production. 14 Using a self-generated atmosphere, FeF3.3H2O was decomposed into a HTB-structured hydroxyfluoride compound FeF3-x(OH)x.nH2O (n ≤ 0.33), denoted FeF3- 15 This sample was heat treated at 250 °C for 12 h, and 350 °C for 1 h to form FeF3-xOx/2ox/2 with x~0.4. X-ray diffraction confirmed the phase purity of the vacancy-containing HTB structure ( Figure S1). The presence of anionic vacancies was ascertained by 57 Fe Mössbauer spectroscopy through the characteristic signature of five-fold coordinated iron ( Figure S2). 10 In agreement with our previous studies, 10,16 the implementation of anionic vacancies led to a significant improvement of the electrochemical properties as compare to the defect-free compound FeF3-x(OH)x. Using a similar electrode processing, we showed that FeF3-x(OH)x could only reversibly inserted 0.05 Li + per Fe while introducing vacancies improved the reversibility up to 0.6 Li + per Fe ( Figure S3).
The thermal decomposition of the precursor FeF3.3H2O is associated with a distinctive color change, from the light green color of FeF3.3H2O to a brownish hue for FeF3-xOx/2ox/2. To track the evolution of the optical band-gap, we collected diffuse reflectance UV-visible spectrophotometry data, which were translated into Kubelka-Munk 17 absorption spectra. The resulting data for FeF3.3H2O, FeF3-x(OH)x and FeF3-xOx/2ox/2 are shown in Figure 1. On decomposition, the adsorption edge is slightly red-shifted from FeF3.3H2O (Egap = 4.05 eV) to FeF3-x(OH)x.nH2O (Egap = 3.75 eV). These band-gap energies indicate an electronic insulator behavior, which suggests that substituting F with OH does not significantly alter the electronic properties of this material. 18 After a second thermal treatment to form anionic vacancies, however, the absorption band is more strongly read-shifted, giving an optical band-gap energy for FeF3-xOx/2ox/2 of Egap ≈ 2.05 eV. This large band-gap reduction means this anion-vacant material may be considered a semiconductor, rather than an insulator. This distinction is usually associated with an increase in electronic conductivity, because of the relative ease with which charge carriers can now be thermally excited across the fundamental band gap. The atomic structure of FeF3-xOx/2ox/2 was probed by the pair distribution function (PDF) method obtained by Fourier transformation of the total scattering data. 19 PDF describes structural features across different scales, giving information about local-, intermediate-and long-range order. 20 We refined the structure of FeF3-xOx/2ox/2 using the following procedure: (1) the structural model was refined against the PDF data with the exclusion of the short-range order, i.e., in the 7 Å-50 Å range. In this region, the HTB-type structure (space group: Cmcm) 21  To more clearly visualize the local structure in FeF3-xOx/2ox/2, differential PDF (d-PDF) was obtained by subtracting re-scaled PDFs of FeF3-x(OH)x and FeF3-xOx/2ox/2 ( Figure 2c). The d-PDF allows identification of specific interatomic-distances that appear with the introduction of anionic vacancies at 1.83 Å-2.04 Å, 3.08 Å, 3.40 Å and 6.23 Å. The structural distortion introduced by forming anion vacancies therefore extends beyond the first coordination sphere of iron, which suggests the emergence of new structural motifs. FeF3-xOx/2ox/2 and the corresponding differential PDF (d-PDF).
To better understand the structural changes that accompany dehydroxylation, we performed a series of density functional theory (DFT) calculations, modelling stoichiometric FeF3 (Fe12F36), hydroxylated FeF3 (Fe12F36-x(OH)x), and dehydroxylated FeF3 (Fe12F36-xOx-yoy). We performed calculations at two defect concentrations: the first considered hydroxylation at two anion sites, followed by dehydroxylation to form a single oF+OF defect pair; the second considered hydroxylation at four anion sites, followed by dehydroxylation to form two oF+OF defect pairs.
In both cases, geometry optimization of the dehydroxylated model gives a spontaneous reorganization of Fanions, corresponding to FeF5O + FeF5 → FeF4O + FeF6 changes in Fe coordination environments. Here, we discuss this relaxation in the double vacancy system, while the equivalent details for the single vacancy system are given in the SI.  (Table 1), allowing all distances to be assigned.  The angular-momentum-projected densities of state (pDOS) from our DFT calculations provide information about how defects change the electronic structure of HTB-FeF3 (Figure 4). In Calculated absorption spectra for these three systems show a sequential reduction in the onset of optical absorption in moving from HTB-FeF3 to the hydroxylated system, and then to the dehydroxylated system (Figure 4, bottom). While the reduction in optical band gap predicted by these calculations is not as large as we observe in our experimental absorption spectra (Figure 1), the correct qualitative trend is reproduced, which supports our proposition that this change from insulating to semiconducting behavior is caused by the introduction of lattice oxygen defects.
Mössbauer spectroscopy. Measurements were performed at room temperature using a constant acceleration Halder type spectrometer with a room temperature 57 Co source in transmission geometry. The velocity was calibrated using pure α-Fe as the standard material. Mossbauer spectra were reconstructed using the WinNormos ® software (Wissenschaftliche Elektronik GmbH). Computational methods.

UV-
Our density functional theory (DFT) calculations were performed using the code VASP 25,26 with valence electrons described using a plane-wave basis with a cutoff of 520 eV. Interactions between core and valence electrons were described using the projector augmented wave (PAW) approach 27